Vehicle wheel bearing apparatus is adapted to freely rotatably support a wheel hub for mounting a wheel, via a rolling bearing, and adopted for use with an inner ring rotation type for a driving wheel and both inner ring rotation and outer ring rotation types for a driven wheel. A double row angular ball bearing is widely used in such a bearing apparatus. The double row angular ball bearing has a desirable bearing rigidity, high durability against misalignment and small rotation torque required for fuel consumption. The double row angular contact ball bearing has a plurality of balls interposed between a stationary ring and a rotational ring. The balls contact the stationary and rotational rings at a predetermined contact angle.
The vehicle wheel bearing apparatus is broadly classified into four generation structures. In the first generation type, a wheel bearing with a double row angular contact ball bearing is fit between a knuckle forming part of a suspension and a wheel hub. In a second generation type, a body mounting flange or a wheel mounting flange is formed directly formed on the outer circumference of an outer member. In a third generation type, one of the inner raceway surfaces is formed directly on the outer circumference of the wheel hub. In a fourth generation type, the inner raceway surfaces are formed directly on the outer circumferences of the wheel hub and the constant velocity universal joint. In the description below, the term “outer side” (left hand side in the drawings) of the apparatus denotes a side that is positioned outside of the vehicle body. The term “inner side” (right hand side in the drawings) of the apparatus denotes a side that is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.
In prior art wheel bearing apparatus formed with double row rolling bearings, the bearing arrangements in both the left and right rows are the same. Thus, it has a sufficient rigidity during straight way running; however, optimum rigidity cannot always be obtained during curved way running. That is, the positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts substantially on the middle between the rows of the bearing balls during straight way running. During curve running, larger radial loads and larger axial loads are applied to axles of the vehicle on the side opposite to a curving direction (i.e. axles of the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity in the bearing row of the outer side than that of the bearing row of inner side. This improves the durability and strength of the bearing apparatus. Thus, a known vehicle wheel bearing apparatus is shown in FIG. 11 that can have a high rigidity without enlargement of the bearing apparatus.
The vehicle wheel bearing apparatus 50 is formed by a double row angular ball bearing. An outer member 51 is integrally formed on its outer circumference with a body mounting flange 51c to be mounted on a knuckle (not shown) of a vehicle. Its inner circumference includes double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with a wheel mounting flange 53 integrally formed at one end to mount a wheel (not shown). One inner raceway surface 52a is formed on its outer circumference opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. Its outer circumference includes the other inner raceway surface 54a opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row balls 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the balls 56, 57.
The inner ring 54 is axially immovably secured by a caulked portion 52c formed by plastically deforming the cylindrical portion 52b of the wheel hub 52 radially outward. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55. The seals prevent leakage of grease contained within the bearing apparatus and the entering of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer side ball group 56 is set larger than a pitch circle diameter D2 of the inner side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than that of the inner raceway surface 54a of the inner ring 54. The outer raceway surface 51a of the outer side of the outer member 51 is larger than that of the outer raceway surface 51b of the inner side of the outer member 51. Also the number of outer side balls 56 is larger than the number of the inner side balls 57. The pitch circle diameter D1 of the outer side is set larger than the pitch circle diameter D2 of the inner side (D1>D2). Thus, it is possible to obtain a large rigidity for the bearing apparatus 50 and thus to extend its life. Patent Document 1: Japanese Laid-open Patent Publication No. 108449/2004.
In the prior art bearing apparatus 50, the pitch circle diameter D1 of the outer side ball group 56 is set larger than the pitch circle diameter D2 of the inner side ball group 57. Also, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than that of the inner raceway surface 54a of the inner ring 54. Thus, it is possible to increase the rigidity of the outer side bearing row and to extend the life of the wheel bearing apparatus 50. However, since both the outer member 51 and the wheel hub 52 are enlarged at their outer sides, this inevitably increases their weight, by the enlargement of their diameter, and thus a reduction of weight of the wheel bearing apparatus is limited.
On the contrary, if the wall thickness of the outer member 51 is excessively reduced to reduce the weight of the outer member 51, a problem is caused with the generation of quenching cracks when the outer raceway surfaces 51a, 51b are formed with hardened layers by high frequency induction quenching. Accordingly, it is necessary to determine the limit of the wall thickness of the outer member in consideration of an increase of the strength and durability as well as a reduction of the weight and size of the outer member.
In addition, in the prior art wheel bearing apparatus 50, the outer member 51 and the wheel hub 52 are manufactured from bar members as blank and go through various machining processes such as forging, turning, heat treatment, grinding, super finishing, etc. For example, as shown in FIG. 12, an outline of the outer member 51 is initially forged. Predetermined grinding allowance regions remain on an inner side face of the body mounting flange 51c and an inner side outer circumferential surface contacting the knuckle to an inner circumferential surface (shown by two-dot chain lines).
In addition, the outer member 51 is formed with the double row outer raceway surfaces 51a, 51b having different groove diameters. The groove shoulders 62, 63 are ground to predetermined dimensions in order to prevent a so-called “shoulder riding-over” where oval lines of contact of the balls 56, 57 ride over and derail from the outer railway surfaces 51a, 51b when the moment load is applied to the wheel bearing apparatus. The dimension of the groove shoulder 63 is strictly defined since influence of the moment load caused on a corner portion (boundary) between inner side outer raceway surface 51b and the groove shoulder 63 is larger than that caused on the outer side outer raceway surface 51a. Thus, an edge load would be caused by riding-over of the balls 56, 57. In this specification the term “edge load” means an excessive stress concentration generated on a corner of a member that would cause a premature peeling on a surface of the member.
The turning of the groove shoulders 62, 63 of the outer member 51 not only causes a material loss to the outer member 51 but increases the machining steps and thus increases the manufacturing cost. In addition, since the groove shoulder 63 is formed by punching during the forging process, it is a subject on how material loss of a blank can be reduced.
In the wheel bearing apparatus 50, a left and right asymmetric bearing arrangement exist. Thus, the inside space volumes are different in the inner and outer side bearing rows. Grease is evenly charged in both the inner and outer side bearing rows despite the difference in their inside space volumes. The inner side bearing row has a smaller inside space volume and tends to be filled with an excessive amount of grease. Such an excessive amount of grease causes grease leakage which deteriorate the effectiveness of a brake and increases rotational torque of the wheel bearing apparatus lowering the fuel consumption of the vehicle.